Fail fast when CL_DATABASE_URL is unset in test helpers

[mostlyconsistent]

NewSqlxDB reported the missing env var with t.Errorf and then returned a nil *sqlx.DB. Errorf is log + Fail, which marks the test failed but keeps it running, so the caller's first query dereferenced nil and the run died as:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0]
github.com/jmoiron/sqlx.(*DB).QueryxContext(0x0, ...)

The actual explanation was scrolled off above the stack trace, so a missing local Postgres looked like a crash in sqlx. Use require.NotEmpty, which halts via FailNow and reports the calling

[github-actions]

📊 API Diff Results

No changes detected for module github.com/smartcontractkit/chainlink-evm

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